The present invention relates to a technology of synchronization detection of a digital signal, and more particularly, to a technology of detection of a sync signal included in a digital TV (DTV) signal transmitted from a broadcasting station.
Initially, digital broadcasting was developed mainly for satellite broadcasting. In recent years, however, the trend of digitization has also surged into terrestrial broadcasting. In terrestrial broadcasting, a DTV signal transmitted from a broadcasting station undergoes a greater distortion on a transmission path than in satellite broadcasting.
FIG. 19 illustrates an example of multipath interference typical in terrestrial broadcasting, which is one kind of distortion a signal undergoes on a transmission path. In FIG. 19, a broadcasting station 1901 transmits a DTV signal, and an antenna 1903 for a DTV receiver 1904 receives the DTV signal transmitted from the broadcasting station 1901. The reference numeral 1902 is an obstacle reflecting the DTV signal transmitted from the broadcasting station 1901.
Among DTV signals received by the antenna 1903, a DTV signal directly received from the broadcasting station 1901 in good reception state is a desired signal. For example, a DTV signal W1 is a desired signal. A DTV signal received under bad reception conditions, such as being reflected from the obstacle 1902, is a delayed signal. For example, a DTV signal W2 is a delayed signal. In general, a delayed signal includes a signal delayed in phase behind a desired signal because the transmission distance is longer than that of the desired signal, and a signal reduced in amplitude due to loss of part of energy during reflection from the obstacle 1902.
The antenna 1903 receives both the desired signal W1 and the delayed signal W2. Therefore, a DTV signal input into the DTV receiver 1904 is a composite signal of the desired signal W1 and the delayed signal W2, and thus has a distortion. This phenomenon that the received signal has a distortion because it is a composite of signals received via a plurality of transmission paths is called multipath interference.
FIG. 20 shows a received signal subjected to multipath interference. In FIG. 20, a received signal DT1 is a distorted composite signal of the desired signal W1 and the delayed signal W2.
A digital demodulation device of the DTV receiver 1904 attempts to decrease the distortion of the received signal to demodulate a DTV signal of the desired signal. FIG. 21 shows a configuration of a conventional digital demodulation device, which includes a synchronization detector 101, a waveform equalizer 102 and an error corrector 103. The synchronization detector 101 detects synchronization of the received signal DT1. The waveform equalizer 102 reduces the distortion of the received signal DT1 based on a sync timing signal DT2 output from the synchronization detector 101. The error corrector 103 corrects a code error of a DTV signal DT3 waveform-equalized by the waveform equalizer 102. Data DT4 error-corrected by the error corrector 103 is output to a circuit subsequent to the digital demodulation device in the DTV receiver 1904, to be subjected to processing for image display.
A DTV signal is essentially composed of predetermined data units (segments) each headed by a sync signal representing the timing for synchronization (A or A′ in FIG. 20). The synchronization detector 101 detects the sync signal from the received signal DT1 and outputs the sync timing signal DT2 at the same timing as that of the sync signal.
Synchronization detection is an especially important function in determining the performance of the DTV receiver 1904. Currently, fierce competition on the performance of the DTV receiver is underway, and thus further improvement in performance is requested.
The synchronization detector 101 normally captures the sync signal included in the desired signal W1 (A in FIG. 20) for detection of synchronization (2001 in FIG. 20). However, if the amplitude of the delayed signal W2 included in the received signal DT1 is particularly large, the sync signal included in the delayed signal W2 (A′ in FIG. 20) may be captured (2002 in FIG. 20).
If the synchronization detector 101 detects the sync signal of the delayed signal W2, that is, if a synchronization error occurs, the sync timing signal DT2 is output at the timing of the sync signal included in the DTV signal of the delayed signal W2. The wave equalizer 102 operates based on this sync timing signal DT2 and thus attempts to remove the desired signal W1. However, it is difficult to completely remove the desired signal W1 because the energy of the desired signal W1 is greater than that of the delayed signal W2. As a result, the waveform-equalized DTV signal DT3 includes too many errors to be corrected by the error corrector 103.